Ultrafast laser-induced phase-change structures in silicon

Abstract

Silicon can be considered as one of the pillars the electronics industry has been built on. It owes this position in part to the existence of two structurally different solid phases, having very different physical properties. Upon pulsed laser irradiation, silicon can re-solidify either in the crystalline or amorphous phase, depending on the local supercooling achieved. While this potential has been identified decades ago, it has been mostly employed in applications requiring the transformation of large areas, for instance for the fabrication of solar cells or organic light emitting diode displays. Little work has been done to imprint small phase-change structures.

In this talk, we will provide an overview of our work on amorphous-crystalline micro- and nanostructures written by ultrashort laser pulses. To this end we employ two different strategies. The first one is based on a laser-induced self-organization mechanism that leads to the formation of so-called Laser Induced Periodic Surface Structures (LIPSS). LIPSS are a universal phenomenon observed in metals, semiconductors and dielectrics upon irradiation with multiple short and ultrashort laser pulses. They manifest as self-assembled sub-wavelength periodic surface structures with different symmetries that depend on the processing parameters and are typically formed via ablation. In our case, we are able to prevent ablation and fabricate periodic phase-change structures whose lateral dimensions and thickness can be controlled at will. The second strategy consists in the fabrication of surface ¿ depressed annular amorphous rings with a central crystalline disk. We show that these rings can be scaled in size and stitched together to form arrays with different symmetries, with their unit cells not being confined to circular symmetry. For both fabrication approaches we have studied the dynamics of the phase transformation using femtosecond-resolved microscopy, which enables the identification and duration of the different processes involved, including free electron generation, thermal and non-thermal melting, liquid phase overheating and rapid solidification into the amorphous phase. The applicability of the writing strategy and the monitoring technique to other materials is discussed, together with potential applications.